Transmit power control is of outmost importance in wireless communication systems.
In many communication systems so-called fast power control is employed for the uplink. Basically, as schematically illustrated in FIG. 1, this implies that a base station 10 measures the received channel quality, e.g. in terms of Signal-to-Interference Ratio, SIR, or Signal-to-Interference-plus-Noise Ratio, SINR, from each user equipment, UE, 20 and commands each UE to adjust its transmit power accordingly.
Fast power control is often referred to as Inner Loop Power Control, ILPC, and is commonly used, e.g. in wireless communication systems based on Code Division Multiple Access, CDMA, such as W-CDMA and CDMA2000.
A target for conventional ILPC is normally to ensure that the received SIR or SINR is at an appropriate level for each UE. If the received SIR or SINR is below target, i.e. too low for proper demodulation and/or decoding, the base station will normally command the UE to increase the UE transmit power. If the received SIR or SINR is above target, the base station commands the UE to decrease the UE transmit power. The SIR or SINR target is typically set by the so-called Outer Loop Power Control. Without power control in the system, the inter-user-interference would make it impossible for the base station to decode transmissions from some users.
According to standard specifications such as 3GPP TS 25.214, the UE's transmission power should basically be updated every slot based on a signal quality measurement performed by the radio base station. The base station therefore generates Transmit Power Control, TPC, commands for the UE based on signal quality estimates, where each TPC command normally indicates a step-size (e.g. 1 dB) transmit power increase/decrease. This is sometimes referred to as the base line TPC command generation. A TPC command indicating a transmit power increase (e.g. 1 dB) is normally referred to as a power-up command, and a TPC command indicating a transmit power decrease (e.g. 1 dB) is normally referred to as a power-down command.
However, when the system is close to its capacity, stability is reduced and so called power rushes can occur because one or several of the users can not reach their SIR or SINR targets. This is also referred to as the “party effect”, where users tries to “talk” louder and louder as the general level of interference increases. This is a very significant problem in many systems such as WCDMA since the power control loops are very fast and capable of stepping up the UE power with up to 1500 dBs/second (1 dB step size 1500 times per second).
Experience also shows that conventional uplink power transmit control mechanisms often lead to fluctuations in SIR or SINR, which affect the Rise-over-Thermal noise (RoT). RoT is normally defined as the ratio between the total power received from all UEs on one hand and thermal noise on the other, and is often used as a measure to indicate “congestion” or “overload”. In modern communication systems, such as WCDMA, the uplink may be non-orthogonal by design, and the capacity and coverage is limited by the maximum RoT. In scenarios with few users transmitting simultaneously, e.g. a few high-rate users or a few users operating in Time Division Multiplexing, TDM, fashion, the RoT level can change very rapidly often due to the SIR fluctuations of individual users.
There are attempts to limit SIR variations, e.g. in the standard specification 3GPP TS 25.214, but the often more important stability control is still an issue and strict control of maximum RoT is important.
The state-of-the-art solutions for uplink transmit power control do not provide optimal performance with respect to stability, capacity, and/or coverage.